The website of the U.S. Census Bureau provides state maps featuring county lines for all U.S. states, including Illinois. Visitors may click on individual counties to view census data and further information about each county.
The official website of the State of Illinois also provides information about the counties in the state. It provides a link to the Illinois Blue Book, which has a wide range of data, in PDF format, that can be viewed online or downloaded. The book includes a map of counties, their administrative centers and a detailed list of the cities, towns and villages of Illinois, complete with population information.